How Much Solar Does My Business Need?

Start with what you use in the day

MCS defines self-consumption as the electricity generated on site that is used in the building and not exported (MIS 3002). That is the part that does the work. DESNZ's second quarter 2026 figures put electricity for a business using 20 to 499 MWh a year at 28.6p per kWh including the Climate Change Levy (DESNZ), while our examples use a 4.1p export rate. Ofgem says export rates are set by suppliers and must be above zero (Ofgem). A unit used is worth several units exported.

So the first question isn't "how many panels fit?" but "how much power do we use between morning and late afternoon, on which days, in which months?"

Using 12 months of half-hourly data

Half-hourly data shows your consumption in each half hour of each day. With 12 months of it, an installer can lay an expected solar output over your real use and see how much would be used on site and how much exported, season by season. Ask your electricity supplier for it; businesses with half-hourly or smart meters can usually get it as a spreadsheet.

• Daytime base load. What you use every weekday hour from morning to afternoon: lighting, IT, refrigeration, compressors, machinery.
• Weekends and closures. A business shut at weekends exports most of what it makes on Saturdays and Sundays.
• Summer and winter. Solar makes far more in summer. A site that uses more in summer (cooling, harvest, holiday season) suits it well.
• Peaks. Short spikes matter less to solar than the steady daytime load.

How much a system makes

Yearly generation is roughly the system size in kWp times the yield for the site. The PVGIS yields in our payback calculator run from about 810 kWh per kWp in the Highlands and North Wales to about 1,160 on the south coast (PVGIS 5.3). The roof's direction, tilt and shading then move that up or down.

Compare that with your yearly daytime use, not your total use. More on sizes: commercial solar system sizes.

Self-consumption vs export: why bigger isn't always better

The two worked examples on our payback calculator show the effect. They are examples only, with the assumptions stated:

The farm's bigger system makes far more electricity, but it uses less than half of it, so its payback is two years longer. Sized closer to its daytime use, a smaller system would have used a larger share. See commercial solar payback.

Roof space and your electricity supply set the ceiling

Once the right size is known, two things can cap it. The first is space: the usable roof or land after edges, rooflights, plant and shading. The installer lays the panels out on a drawing of your roof.

The second is the grid. Systems up to 16A per phase connect under G98 with a notification; anything larger goes through G99, where the network operator may limit what you can export (G98, G99). A system sized to your daytime use exports less, which can make that easier.

When a battery changes the answer

If your data shows a regular daytime surplus and steady use after the panels stop, a battery can store one for the other and raise the share you use yourself. If you use almost everything as it is made, a battery adds cost and little else. See commercial battery storage.